Convergent charged particle beam apparatus and inspection method using same

ABSTRACT

A convergent charged particle beam apparatus includes an electron beam system which emits a converged electron beam, a vacuum chamber which is connected to the electron beam system, and a stage which mounts a specimen and moves at least in one direction inside of the vacuum chamber. An electron beam image observation unit observes an electron beam image of a surface of the specimen. A height detector optically detects a height of the specimen mounted on the stage by illuminating the surface of said specimen with light and by detecting reflected light of the illumination reflected from the surface of the specimen, and a controller controls a focus position of the electron beam in accordance with an output from the height detector while the stage is moving at least in one direction.

CROSS REFERENCE TO RELATED APPLICATION

This application a continuation application of U.S. application Ser. No.10/426,702, filed May 1, 2003, now U.S. Pat. No. 6,744,057, which is acontinuation of U.S. application Ser. No. 10/012,400, filed Dec. 12,2001, now U.S. Pat. No. 6,559,459, which is a continuation of U.S.application Ser. No. 09/258,461, filed Feb. 26, 1999, now U.S. Pat. No.6,335,532, which is a continuation-in-part application of U.S.application Ser. No. 09/132,220, filed Aug. 11, 1998, by some of theinventors herein, now U.S. Pat. No. 6,107,637, the subject matter of theaforementioned applications being incorporated by reference herein.

BACKGROUND OF THE INVENTION

The present invention relates to a convergent charged particle beamapparatus using a charged particle beam such as an electron beam or ionbeam for microstructure fabrication or observation and an inspectionmethod using the same, and more particularly to an automatic focusingsystem and arrangement in the convergent charged particle beamapparatus.

As an example of an apparatus using a charged particle beam, there is anautomatic inspection system intended for inspecting and measuring amicrocircuit pattern formed on a substrate such as a semiconductorwafer. In defect inspection of a microcircuit pattern formed on asemiconductor wafer or the like, the microcircuit pattern under test iscompared with a verified non-defective pattern or any correspondingpattern on the wafer under inspection. A variety of optical micrographimaging instruments have been put to practical use for this purpose, andalso electron micrograph imaging has found progressive applications todefect inspection by pattern image comparison. In a scanning electronmicroscope instrument which is specifically designed forcritical-dimension measurement of line widths and hole diameters onmicrocircuit patterns used for setting and monitoring process conditionsof semiconductor device fabrication equipment, automaticcritical-dimension measurement is implemented through use of imageprocessing.

In comparison inspection where electron beam images of correspondingmicrocircuit patterns are compared for detecting a possible defect or incritical-dimension measurement where electron beam images are processedfor measuring such dimensions as pattern line widths, reliability ofresults of inspection or measurement largely depends on the quality ofelectron beam images. Deterioration in electron beam image qualityoccurs due to image distortion caused by deflection or aberration inelectron optics, decreased resolution caused by defocusing, etc.,resulting in degradation of performance in comparison inspection orcritical-dimension measurement.

In a situation where a specimen surface is not uniform in height, ifinspection is conducted on the entire surface area under the samecondition, an electron beam image varies with each region inspected asexemplified in FIGS. 1(a)-1(d), wherein FIG. 1(a) shows a wafer withdifferent regions A-C, FIG. 1(b) shows an in-focus image of region A andFIGS. 1(c) and 1(d) show defocused images of regions B and C,respectively. In inspection by comparison between the in-focus image ofFIG. 1(b) and the defocused image FIG. 1(c) or FIG. 1(d), it isimpossible to attain correct results. Further, since these imagesprovide variation in pattern dimensions and results of edge detection onthem are unstable, pattern line widths and hole diameters cannot bemeasured accurately. Conventionally, image focusing on an electronmicroscope is performed by adjusting a control current to an objectivelens thereof while observing an electron beam image. This procedurerequires a substantial amount of time and involves repetitive scanningon a surface of a specimen, which may cause a possible problem ofspecimen damage.

In Japanese Non-examined Patent Publication No. 258703/1993, there isdisclosed a method intended for circumventing the abovementioneddisadvantages, wherein an optimum control current to an objective lensfor each surface height of a specimen is pre-measured at some points onthe specimen and then, at the time of inspection, focus adjustment ateach point is made by interpolation of pre-measured data. However, thismethod is also disadvantageous in that a considerable amount of time isrequired for measuring an optimum objective lens control current beforeinspection and each specimen surface height may vary during inspectiondepending on wafer holding conditions.

A focus adjustment method for a scanning electron microscope using anoptical height detecting arrangement is found in Japanese Non-examinedPatent Publication No. 254649/1988. However, since an optical elementfor height detection is disposed in a vacuum system, it is ratherdifficult to perform optical axis alignment.

In microstructure fabricating equipment using a convergent chargedparticle beam, focus adjustment of the charged particle beam has asignificant effect on fabrication accuracy, i.e., focus adjustment is ofextreme importance as in instruments designed for observation. Examplesof microstructure fabricating equipment include an electron beamexposure system for forming semiconductor circuit patterns, a focusedion beam (FIB) system for repairing circuit patterns, etc.

In a scanning electron microscope, a method of measuring an optimumcontrol current to an objective lens thereof through electron beamimaging necessitates attaining a plurality of electron beam images fordetecting a focal point, thus requiring a considerable amount of timefor focus adjustment. That is, such a method is not suitable forfocusing in a short time. Further, in an application of automaticinspection or critical-dimension measurement over a wide range, focusadjustment at every point using the abovementioned method is notpracticable, and it is therefore required to perform pre-measurement atsome points before inspection and then estimate a height at each pointthrough interpolation, for instance. FIG. 2 shows an overview of anelectron-beam automatic semiconductor device inspection system to whichthe present invention is directed. In such an automatic inspectionsystem, a specimen wafer under inspection is moved by means of stageswith respect to an electron optical system thereof for carrying outwide-range inspection.

A semiconductor wafer to be inspected in a fabrication process maydeform due to heat treatment or other processing, and a degree ofdeformation will be on the order of some hundreds of micrometers in theworst case. However, it is extremely difficult to hold the specimenwafer stably without causing interference with electron optics in avacuum specimen chamber, and also it is impossible to adjust specimenleveling as in an optical inspection system using vacuum chucking.

Further, since a substantial amount of time is required for inspection,a specimen holding state may vary due to acceleration/deceleration inreciprocating stage movement, thereby resulting in a specimen surfaceheight being different from a pre-measured level.

For the reasons mentioned above, there is a rather high degree ofpossibility that a surface height of a specimen under inspection willvary unstably exceeding a focal depth of the electron optical system (adepth of focus is generally on the order of micrometers at amagnification of 100×, but that necessary for semiconductor deviceinspection depends on inspection performance requirements concerned).For focus adjustment using electron beam images, a plurality of electronimages must be attained at each point of interest with each stage beingstopped. It is impossible to conduct focus adjustment continuously whiledetecting a height at each point simultaneously with stage movement forthe specimen under inspection.

In an approach that focus adjustment using electron beam images isperformed at some points on a specimen surface before the start ofinspection, an amount of time is required for calibration beforeinspection. This causes a significant decrease in throughput as a sizeof wafer becomes larger. Since there is a technological trend towardlarger-diameter wafers, a degree of wafer deformation such as bowing orwarping will tend to be larger, resulting in more stringent requirementsbeing imposed on automatic focusing functionality. Depending on thematerial of a specimen, exposure with an electron beam may alter anelectric charge state on specimen surface to cause an adverse effect onelectron beam images used for inspection.

In consideration of the above, it is difficult to ensure satisfactoryperformance in long-period inspection on a scanning electron microscopeinstrument using the conventional methods. Where stable holding of aspecimen is rather difficult, it is desirable to carry out specimensurface height detection in a range of electron optical observationimmediately before images are attained during inspection. Further, whereinspection is conducted while each stage is moved continuously, specimensurface height detection must also be carried out continuously at highspeed without interrupting a flow of inspection operation. For realizingcontinuous surface height detection simultaneously with inspection, itis required to detect a height of each inspection position or itsvicinity at high speed.

However, if any element which affects an electric or magnetic field,e.g., an insulating or magnetic element, is disposed in the vicinity ofan observation region, electron beam scanning is affected adversely. Itis therefore impracticable to mount a sensor in the vicinity of electronoptics. Further, since the observation region is located in the vacuumspecimen chamber, measurement must be enabled in a vacuum. For use inthe vacuum specimen chamber, it is also desirable to make easyadjustment and maintenance available. While there have been describedconditions as to an example of an electron-beam inspection system, theseconditions are also the same in a microstructure observation/fabricationsystem using an ion beam or any other convergent charged particle beam.Further, since there are the same conditions in such systems that imagesof an aperture, mask, etc. are formed or projected as well as in asystem where a charged particle beam is converged into a single point,it is apparent that the present invention is applicable to chargedparticle beam systems comprising any charged particle beam optics forimage formation/projection.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide anarrangement for detecting a surface height of an object item in anobservation/fabrication region of charged particle beam optics or in itsvicinity under vacuum at high speed without causing interference withthe charged particle beam optics.

According to the present invention, there is provided a highly reliablesystem in which the object item can be observed/fabricated with itsimage being always in focus using surface height data thus detected.

According to an embodiment of the present invention, there is provided aheight detector capable of detecting a surface height of an object itemin an observation/fabrication region without causing interference withcharged particle beam optics simultaneously with observation/fabricationand a system capable of carrying out observation/fabrication using acharged particle beam image formed in the charged particle beam opticsin which focus adjustment can be made with height data obtained throughthe height detector. For enabling specimen height detection withoutcausing interference with the charged particle beam optics, it isnecessary to provide a height detector which can detect a surface heightfrom a distant position apart from the charged particle beam optics.Further, for preventing an adverse effect on charged particle beamscanning, a height detection method must be arranged so that influenceon electric and magnetic fields in the vicinity of a detection positionwill not vary with time. Additionally, since a specimen chamber isevaluated, the height detector must be usable under vacuum.

According to one aspect of the present invention, there is provided aheight detector based on an optical height detection method in whichlight is projected to a height detection position slantwise andreflected light from a specimen surface is measured for heightdetection.

In accordance with the present invention, a convergent charged particlebeam system comprises an electron beam source, an electron opticalsystem unit for converging an electron beam emitted from the electronbeam source into focus, a vacuum chamber unit having the inside thereofevacuated, a stage unit arranged in the inside of the vacuum chambermeans so as to mount a specimen under inspection thereon and move thespecimen along each plane, an electron beam image observation unit forobserving an electron beam image of a surface of the specimen mounted onthe stage unit in a manner that the electron beam converged by theelectron optical system means is scanned over the surface of thespecimen for irradiation and secondary charged particles produced fromthe specimen are detected, a height detecting unit for opticallydetecting a specimen surface height in a region irradiated with theelectron beam scanned by the electron optical system means, a controlunit for controlling a focal point of the electron beam converged by theelectron optical system unit and a heightwise relative position of thespecimen through use of resultant data detected by the height detectingunit, and a defect detecting unit for detecting a possible defect on thespecimen by processing electron beam image data of the surface of thespecimen observed by the electron beam image observation means in astate that the focal point of the electron beam and the heightwiserelative position of the specimen are controlled by the control unit.

Further, in accordance with the present invention, there is provided aninspection method using a convergent charged particle beam system,comprising the steps of setting a specimen under inspection on a movabletable inside a processing chamber, evacuating the processing chambercontaining the specimen, scanning an electron beam emitted from anelectron beam source while moving the movable table in the inside of theevacuated processing chamber to optically detect a height of ascanning-electron-beam-irradiated region on a surface of the specimen inan optical axis direction of the electron beam source, adjusting asurface height of the specimen in the optical axis direction accordingto resultant height data thus detected, scanning the electron beamemitted from the electron beam source for irradiation over the specimenthus adjusted in height while moving the movable table, detectingsecondary charged particles produced from the specimen irradiated withthe electron beam through scanning to attain a secondary chargedparticle image of the surface of the specimen, and inspecting thespecimen using the secondary charged particle image thus attained.

These and other objects, features and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a)-1(d) show inspection of a wafer at different regions andelectron beam images of the different regions;

FIG. 2 is a schematic sectional view showing an exemplary structure ofan automatic inspection system according to the present invention;

FIG. 3 is a schematic sectional view of a height detection opticalsystem for illustrating a principle of height detection;

FIG. 4 is a graph showing variation in reflectance with respect toincidence angle on each material;

FIG. 5 is a schematic sectional view of a specimen chamber, showing anexample of altered disposition of height detection optical system parts;

FIG. 6 is a schematic sectional view of a specimen chamber, showing anarrangement in which the height detection optical system parts aredisposed outside the specimen chamber;

FIG. 7 is a schematic sectional view of a specimen chamber, showing anarrangement in which the height detection optical system parts aredisposed inside the specimen chamber;

FIG. 8 is a schematic sectional view of a specimen chamber, showing anarrangement in which optical path windows are formed along a plane of anexternal top wall of the specimen chamber;

FIG. 9 is a graph showing variation in reflectance with respect toincidence angle on glass BK7;

FIG. 10 is a schematic sectional view of a specimen chamber, showing anarrangement in which optical path windows are formed perpendicularly toan optical path on an external top wall of the specimen chamber;

FIG. 11 is a schematic sectional view illustrating chromatic aberrationdue to a glass window;

FIG. 12 is a schematic sectional view illustrating an arrangement inwhich a glass plate is inserted for correction of chromatic aberrationdue to a glass window;

FIG. 13 is a schematic sectional view illustrating another arrangementin which a glass plate is inserted in a different manner for correctionof chromatic aberration due to a glass window;

FIGS. 14(a) and (b) are schematic sectional views showing a change inoptical path size on a flat-plate electrode according to incidenceangle;

FIG. 15 is a schematic sectional view showing a shape of an entranceopening on the flat-plate electrode in case of a circular opticalaperture;

FIG. 16 is a schematic sectional view showing a shape of an entranceopening on the flat-plate electrode in case of an elliptical opticalaperture;

FIG. 17 is a schematic sectional view showing an example of an windowformed perpendicularly to an optical path on the flat-plate electrode;

FIG. 18 is a schematic top view showing an example of disposition inwhich a window is provided in a circumferential form symmetrically withrespect to an optical axis of an electron beam optical system;

FIG. 19 is a schematic top view showing an example of disposition inwhich windows are provided symmetrically with respect to an axis ofdeflection direction;

FIG. 20 is a schematic top view showing another example of dispositionin which windows are provided in a parallel form symmetrically withrespect to an axis of deflection direction;

FIG. 21 is a perspective view of a standard calibration pattern having aslope part;

FIG. 22 is a schematic section view showing an automatic inspectionsystem in which the standard calibration pattern is secured to an X-Ystage;

FIG. 23 is a graph for explaining a relationship between objective lenscontrol current and specimen surface height;

FIG. 24 is a perspective view of a standard calibration pattern havingtwo step parts;

FIG. 25 is a schematic sectional view showing an automatic inspection inwhich the standard calibration pattern is mounted on a Z stage;

FIG. 26 shows a relationship between deviation in measurement positionand error in height detection;

FIGS. 27(a) and (b) show views of a specimen surface for explaining amethod of presuming an observation region height using height datadetected continuously;

FIGS. 28(a)-(c) show views of a specimen surface for explaining a methodof presuming an observation region height using height data detectedcontinuously;

FIGS. 29(a) and (b) show views of a specimen surface for explaining amethod of presuming an observation region height using height datadetected continuously in a different manner;

FIG. 30 is a schematic sectional view of a specimen chamber in which aheight detection optical system can be moved in parallel to an electronoptical system;

FIG. 31 is a schematic section view of a specimen for explaining aheight detection error due to nonuniform reflectance on a specimensurface;

FIG. 32 is a schematic sectional view of an optical system in which twoslit light beams are projected symmetrically for detection; and

FIGS. 33(a)-(c) show diagrams for explaining height detection using aplurality of fine slit light beams.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the accompanying drawings wherein like referencenumerals are utilized to designate like parts throughout the views,there is shown in FIG. 2 an overview of an automatic semiconductordevice inspection system using electron beam images as an exemplarypreferred embodiment of the present invention. In an electron opticalsystem shown in FIG. 2, an electron beam emitted from an electron gun 1is converged through an objective lens 2, and the electron beam thusconverged can be scanned over a surface of a specimen in an arbitrarysequence. A signal of secondary electrons 4 produced on a surface of aspecimen wafer 3 in irradiation with the electron beam is detected by asecondary electron detector 5, and then the secondary electron signal isfed to an image input part 6 as an image signal.

The specimen wafer under inspection can be moved by an X-Y stage 7 and aZ stage 8. By moving each stage, an arbitrary point on the surface ofthe specimen wafer is observable through the electron optical system.Electron beam irradiation and image input can be performed insynchronization with stage movement, which is controlled under directionof a control computer 10. A height detector 11 is of an opticalnon-contact type which does not cause interference with the electronoptical system, and it can speedily detect a height of the specimensurface at or around an observation position in the electron opticalsystem by a height calculator 11 a. Resultant data of height detectionis input to the control computer 10.

According to the height of the specimen surface, the control computer 10adjusts a focal point of the electron optical system, i.e., a positionof the Z stage, and it receives input of the image signal. Using theimage signal input in a focused state and inspection position datadetected by a position monitoring measurement device, defect judgment iscarried out through comparison with a pattern pre-stored by an imageprocessing circuit 9, a corresponding pattern at a location on thespecimen wafer under inspection, or a corresponding pattern on adifferent wafer with a defect being detected by defect detector 100.While the automatic semiconductor device inspection system usingsecondary electron images is exemplified in FIG. 2, back scatteredelectron images or transmitted electron images may also be used forspecimen surface observation instead of secondary electron images.

In the example shown in FIG. 2, a spot or slit light beam is projectedonto the specimen surface, reflected light therefrom is imaged, and aposition of a light beam image thus attained is detected for determininga height of the specimen surface (hereinafter referred to as alight-reflected position detecting method). More specifically, as shownin FIG. 3, the spot or slit light beam is projected onto the specimensurface at a predetermined angle of incidence so that its image isformed on the specimen surface, and reflected light thereof from thespecimen surface is detected. Through conversion from specimen surfaceheight variation to light beam image shift, a degree of light beam imageshift is detected to determine a height of the specimen surface.

The height detector described above may also be applicable to differenttypes of microstructure observation/fabrication systems using otherconvergent charged particle beams as in the inspection systemexemplified in FIG. 2. The following exemplary preferred embodiments ofthe height detector are described as related to a microstructureobservation system using a charged particle beam, but it is apparentthat the height detector may also be applicable to a microstructurefabrication system using a charged particle beam. As will be apparent tothose skilled in the art, the degradation in image quality in themicrostructure observation system corresponds to the degradation infabrication accuracy in the microstructure fabrication system. It isalso apparent that the present invention is not limited in itsapplication to a charged particle beam system in which a chargedparticle beam is converged to a single point. The present invention isfurther applicable to such microstructure fabrication systems thatimages of an aperture, mask, etc. are formed/projected, and it providessimilar advantageous effects in these systems having image-formingcharged particle optics. As an example of such microstructurefabrication systems, there is an electron beam lithography system usingcell-projection exposure.

In the light-reflected position detecting method mentioned above, sincea height detection optical element is not located directly above adetection position, a height in an observation region in a chargedparticle beam optical system can be detected simultaneously withobservation by the charged particle beam optical system in a fashionthat virtually no interference takes place. By making a height pointdetected by the height detector meet an observation region in thecharged particle beam optical system, a surface height of an object itemcan be known at the time of observation. In this arrangement, throughfeedback of height data thus attained, observation can be conductedusing a charged particle beam which is always in focus.

It is not necessarily required to provide such a condition that adesired observation region in the charged particle beam optical systemmeets a corresponding height point detected by the height detector, butrather it is just required that a surface height of the object isrecognizable at the time of observation using vicinal height dataattained successively. In use of the light-reflected position detectingmethod, optical parts may be arranged flexibly to some extent in opticalsystem design, and it is therefore possible to dispose the optical partsto prevent interference with the charged particle beam optical system.

Disposition of the height detector in the light-reflected positiondetecting method is substantially limited by an angle of incidence onthe object surface. In the light-reflected position detecting method,since a degree of incidence angle has an effect on height detectionperformance, an incidence angle cannot be determined only by partdisposition in the system. FIG. 4 shows incidence angle dependency ofsurface reflectance of silicon and a resist which are representativematerials used in formation of semiconductor wafer circuit patterns. Avalue of reflectance on specimen surface increases with an increase inincidence angle, and a difference in reflectance between materialsdecreases with an increase in incidence angle. This tendencycharacteristic also holds for other kinds of materials. Any differencein reflectance between materials causes nonuniform reflectance on thespecimen surface, causing irregularity in distribution of the quantityof light detected. If irregular distribution of the quantity of lightoccurs in a detected slit image due to nonuniform reflectance ofspecimen surface pattern, an error takes place in slit positiondetection, resulting in a decrease in accuracy of height detection.

Referring to FIG. 3, a degree of light beam image shift is detected by aposition sensor. Instead of the position sensor, a linear image sensoror any sensor capable of detecting a light beam irradiating position mayalso be used. For ensuring a proper S/N ratio in output of such asensor, it is required to detect an adequate quantity of light. Toprovide a sufficient quantity of light for stable detection, it isdesirable to increase the incidence angle. In principle, detectionsensitivity in the light-reflected position detecting method becomehigher as the incidence angle with respect to the vertical increases. Anadequate quantity of detected light can be ensured by providing anarrangement that the incidence angle is 60 degrees or more. Moreparticularly, it has been determined that 70 degrees provides goodresults.

Exemplary preferred embodiments of disposition of optical parts in aheight detection optical system are described in the followingdescription wherein in general, if an insulator is located in thevicinity of a charged particle beam optical system, a possible chargebuild-up in the insulator affects an electric field around it to causean adverse effect on charged particle beam deflection, resulting indegradation in image quality. Since such a charging effect varies withtime as a charged condition changes, compensation for it is difficultpractically.

For attaining a stable charged particle beam image, disposition of aninsulator such as a lens at a position encountered with the chargedparticle beam must be avoided. If the insulator is coated with aconductive film and disposed at a position sufficiently apart from thecharged particle beam optical system, an adverse effect may be reduced.A degree of requirement for preventing an adverse effect of theinsulator (lens) on the charged particle beam optical system depends onspecifications of the charged particle beam optical system such asvisual field condition, accuracy, resolution, etc. According to thespecifications of the charged particle beam optical system, a rangeinfluential on the charged particle beam optical system may bedetermined, and an optical path may be designed so that the insulator isnot disposed in the influential range, thus preventing an adverse effecton the charged particle beam optical system.

When a lens for the height detector is disposed in the periphery of thecharged particle beam optical system, an effect on the charged particlebeam can be presumed experimentally through computer simulation. Theheight detection optical system may be designed after determining asuitable mounting position of each lens as illustrated in FIG. 5. Adistance between a surface of a specimen (imaging point) and each oflenses 16 and 17 facing the specimen may be adjusted by selecting lenseshaving a proper focal length.

In the preferred embodiment mentioned above, each lens is disposed at aposition which does not cause an adverse effect on the charged particlebeam optical system. Further, as shown in FIG. 6, there may also beprovided such an arrangement that the lenses and other parts of theheight detection optical system can be located outside a vacuum specimenchamber 13 by increasing a distance between the specimen surface andeach lens facing the specimen. On a casing between the inside of thevacuum specimen chamber 13 and the atmosphere, there may be provided atransparent window made of glass or the like. In this arrangementwherein the optical parts of the height detection optical system aredisposed outside the vacuum specimen chamber, adjustment at the time ofinstallation and maintenance thereafter will be easier advantageouslythan when the height detection optical system is disposed in a vacuum asshown in FIG. 7.

As in the preferred embodiment exemplified above, some or all of theoptical parts of the height detection optical system may be arrangedoutside the vacuum specimen chamber. As illustrated in FIG. 8, wheresome or all of the optical parts are disposed outside the vacuumspecimen chamber, an external wall for separation between the inside ofthe vacuum specimen chamber and the atmosphere is located on an opticalpath. For allowing passage of light through the external wall, it isnecessary to provide an entrance window made of transparent materialsuch as glass. In an arrangement that the entrance window is formedalong a plane of the external wall at the top of the vacuum specimenchamber as shown in FIG. 8, if a light beam is projected at a high angleof incidence in the light-reflected position detecting method, anincidence angle of the light beam to the entrance window becomes largerto increase reflectance on a surface of the entrance windowsignificantly.

Referring to FIG. 9, there is shown incidence angle dependency ofsurface reflectance of a representative kind of glass BK7 which iscommonly used as an optical material. Since the surface of the entrancewindow may be coated with a conductive film and different kinds ofwindow materials may be used, the incidence angle dependency will varyto some extent but its tendency characteristic is similar. As theincidence angle to the surface of the entrance window increases, a valueof surface reflectance increases to cause larger loss in the quantity oflight at passage through the entrance window.

As shown in FIG. 8, light may pass through two windows; an entrancewindow when it is projected onto a surface of a specimen, and an exitwindow after it is reflected therefrom. As the number of windows throughwhich light passes is increased, loss in the quantity of light becomeslarger. Further, in consideration of incidence angle distribution in thelight beam (e.g., incidence angle distribution in a range of ±5.7 deg.in case of NA 0.1), it is required to avoid providing an incidence anglewhich causes significant variation in reflectance in order to preventirregular distribution of the quantity of light in the beam.

Accordingly, as shown in FIG. 10, there may be provided such anarrangement that an entrance window 23 is formed perpendicularly to orat an angle which is almost perpendicular to the optical path of theheight detection optical system for reducing surface reflectance on thewindow, thereby decreasing loss in the quantity of light on the opticalpath. In consideration of possible irregularity in distribution of thequantity of light in the beam, it is preferred to dispose the entrancewindow at an incidence angle of 30 deg. or less so that there will occurlittle variation in reflectance with incidence angle as indicated inFIG. 9. In addition to the external wall for separation between theinside of the vacuum specimen chamber and the atmosphere, there may beany member part on the optical path in the height detection opticalsystem. If it is impossible to provide an opening through the memberpart, it is required to arrange a window thereon in the same manner. Insuch a case, loss in the quantity of light can be minimized by forming ashape of the window perpendicularly to the optical path as far aspossible on condition that the shape of the window does not cause anadverse effect on the charged particle beam optical system.

The following description describes exemplary preferred embodiments forreducing an effect of chromatic aberration due to variance in refractiveindex of glass material used for a window for light passage. When alight beam for height detection passes though the window made of glass,its optical path is made to shift. As shown in FIG. 11, since there isvariance in refractive index of glass material, a degree of optical pathshift varies depending on wavelength. When white light is used forspecimen surface height detection, an error may occur in heightdetection due to chromatic aberration caused by the white light.

Further, the degree of optical path shift is dependent on an angle ofincidence and proportional to a thickness of glass plate. If theincidence angle to the glass plate of the window is decreased as in theforegoing preferred embodiment, the degree of optical path shift can bereduced. However, if the incidence angle is rather large, there arises aparticular problem. (For example, in case that the incidence angle is 70deg., glass BK7 is used and the thickness of glass plate is 2 mm, thereoccurs a difference of 9 μm in optical path shift between wavelengths of656.28 nm and 404.66 nm.)

Where white light is used, an effect of chromatic aberration varies withcolor of an object under inspection and therefore its correction israther difficult. For reduction in effect of chromatic aberration, theremay be provided such arrangements that the window glass plate is madethinner and a glass plate for correcting chromatic aberration isinserted on the optical path. Since the degree of optical path shift isproportional to the thickness of window glass plate, it is preferred touse a glass plate having a thickness which will not cause significantchromatic aberration, in consideration of applicable wavelength coverageand desired accuracy of height detection.

It is not necessarily required to use glass material if a requiredstrength can be satisfied, and therefore an optically transparent partmade of pellicle material, for example, may be employed. However, incase of the window on the vacuum specimen chamber, considerable strengthis required and it is not permitted to make the glass plate sufficientlythinner. Therefore, in such a case, the glass plate for correctingchromatic aberration may be inserted on the optical path.

Referring to FIG. 12, there is shown an arrangement that a chromaticaberration correcting glass plate is inserted in the same positionalrelation as that of an entrance window with respect to an imaging lens.In this arrangement, a difference in degree of optical path shift can becanceled by disposing the chromatic aberration correcting glass plate,which has the same characteristic as the entrance glass window in thatit, for example, is made of the same material as that of the entrancewindow and has the same thickness as that of the entrance window, sothat an incidence angle to the chromatic aberration correcting glassplate will be θ with respect to an incidence angle to the entrance glasswindow θ. A similar arrangement may also be provided on the detectorside with respect to the exit glass window.

Further, in FIG. 13, there is shown an arrangement that a chromaticaberration glass plate and an imaging lens are located in reverse. Inthis arrangement, a difference in degree of optical path shift can alsobe canceled by disposing the chromatic aberration correcting glassplate, which is made of the same material as that of the entrance windowand has a thickness proportional to a magnification of the imaging lens,so that the chromatic aberration correcting glass plate will be inparallel to the entrance window.

For the purpose of decreasing an accelerating voltage for the chargedparticle beam to be applied onto a specimen, a flat-plate electrode maybe arranged at a position over a surface of the specimen in parallelthereto. In this arrangement, it is required to provide an opening orwindow on the flat-plate electrode to allow passage of light on anoptical path for the height detector. Since a shape of the flat-plateelectrode has an effect on electric field distribution in the vicinityof the specimen, it may affect the quality of charged particle beamimages adversely. Exemplary embodiments for reducing an adverse effecton the charged particle beam images are described in the followingdescription. A degree of adverse effect on the charged particle beamoptical system varies depending on the size or position of the openingto be provided on the flat-plate electrode. An permissible level ofadverse effect by the opening depends on performance required for thecharged particle beam optical system. When the size of the opening isconsiderably small, its adverse effect may be negligible. Therefore, amethod for reducing the opening size is explained below.

As shown in FIGS. 14(a) and 14(b), when an incidence angle to a surfaceof an object with respect to the vertical is increased from the smallincidence angle of FIG. 14(a) to the relatively large incidence angle ofFIG. 14(b), the size of an optical path going through a plane parallelto the object surface becomes larger even if a numerical aperture (NA)of the optical path of the height detection optical system is constant.Where the optical path goes through an opening on the flat-plateelectrode 25 as in this case, the shape of the opening 26 must beenlarged substantially in the projecting direction of the optical axisto the flat-plate electrode from that shown in FIG. 14(a) to that shownin FIG. 14(b). This gives rise to a problem particularly in a situationwhere the numerical aperture of the optical system is rather large and adistance between the flat-plate electrode and the object surface israther long. A suitable position of the flat-plate electrode isdetermined according to specifications of the charged particle beamoptical system, and it cannot be changed in common applications.Further, it is not allowed to extremely decrease the numerical aperturesince a sufficient quantity of light must be provided for detection.

Reduction of the size of the opening without decreasing the entirequantity of light for detection is described below. Commonly, an opticallens aperture having a circular shape whose center coincides with theoptical axis is employed. According to one aspect of the presentinvention, there is provided an elliptic or rectangular optical lensaperture having its major axis which is in the axial direction acrossthe optical axis and parallel to the object surface and having its minoraxis which is in the axial direction across the major axis and theoptical axis. In this arrangement, the entire quantity of lightnecessary for height detection can be ensured by providing an ellipticor rectangular area which is equal to that of a circular lens aperture.

FIG. 15 shows an optical geometry of an optical path going through theopening 26 of the flat-plate electrode 25 in case of a circular opticalaperture, and FIG. 16 shows an optical geometry of an optical path goingthrough the opening 26 of the flat-plate electrode 25 in case of anelliptical optical aperture which has almost the same area as that ofthe circular optical aperture in FIG. 15. As can be seen from thesefigures, the size of the opening 26 in one direction on the flat-plateelectrode 25 can be reduced by using the elliptic aperture. Asillustrated here, the size and shape of the opening can be changed bymodifying the shape of the aperture as far as performance required forthe height detector can be ensured. Thus, a degree of adverse effect onthe charged particle beam optical system can be reduced.

If the charged particle beam optical system is affected by the size ofthe opening so that performance required for it cannot be attained, itis necessary to provide a further measure. For example, instead ofmerely a hollow opening formed on the flat-plate electrode, there may beprovided such an arrangement that a window made of glass coated with aconductive film or other material is formed on the flat-plate electrodeto allow passage of light on an optical path. In this arrangement, anadverse effect due to electric field to be given to an object or itsperiphery can be reduced. As exemplified in FIG. 8, if the window isformed at the position of the opening along a plane of the flat-plateelectrode in FIG. 14, significant loss in the quantity of light occursdue to reflection on a surface of the window, causing irregulardistribution in the quantity of light in the beam. Therefore, asexemplified in FIG. 10, there may be provided such an arrangement thatthe window is formed perpendicularly to or at an angle almostperpendicular to the optical path. Thus, loss in the quantity of lightdue to reflection on the surface of the window can be decreased. FIG. 17shows an example of the window formed in this arrangement.

The opening or window formed on the flat-plate electrode in theforegoing examples has a considerable effect on electric potentialdistribution in the vicinity of the object. The following describes anopening/window disposition method for reducing this effect. Since thewindow and opening can be disposed in the same manner, the window istaken in the description given below.

In a microstructure observation/fabrication system to which the presentinvention is directed, two-dimensional observation or fabrication ismostly carried out through two-dimensional scanning by deflecting aconvergent charged particle beam or through stage scanning bycombination of one-dimensional scanning based on charged particle beamdeflection and stage movement in the direction orthogonal to theone-dimensional scanning. According to the present invention, the windowis disposed in consideration of charged particle beam deflection andstage movement direction in charged particle beam scanning. Thus, aneffect of variation in electric field due to the window can be reducedas proposed below.

Referring to FIG. 18, there is shown an example of disposition in whichthe window 29 is provided in a circumferential form having its center atthe optical axis of the charged particle beam optical system. Since thewindow is located at a position apart from a scanning range of thecharged particle beam, an effect of variation in electric field due tothe window is isotropic in the disposition shown in FIG. 18. Thus, theeffect will be almost uniform in an observation region in the chargedparticle beam optical system. Further, it is possible to attain almostthe same result by disposing dummy windows 30 at axisymmetric positionswith respect to the directions of electron beam deflection and stagemovement as shown in FIG. 19.

In case of stage scanning, electric field distribution in a deflectionrange can be made uniform by disposing windows 29 in parallel to thedeflection direction as shown in FIG. 20. If electric field distributionis kept uniform, scanning position correction is allowed to enableimprovement in image quality. In carrying out the present invention, aneffect to be given by the shape and disposition of these windows oropenings is to be examined in consideration of specifications of thecharged particle beam optical system and desired inspection performanceto select suitable window formation and disposition.

The following describes exemplary embodiments for charged particle beamfocus adjustment using height detection result data attained by theheight detector. A focal point of the charged particle beam is adjustedby an objective lens control current. Using input data of an objectsurface height detected by the height detector in an observation regionof the charged particle beam optical system, the objective lens controlcurrent is regulated to enable observation of a charged particle beamimage which is always in focus. For this purpose, in the chargedparticle beam optical system, a level of objective lens control currentis to be calibrated beforehand with respect to variation in objectsurface height. Further, an offset and gain in relation between theheight detector and the charged particle beam optical system are to becalibrated beforehand.

Calibration methods for offset and gain will be described in thefollowing exemplary embodiments. When the charged particle beam opticalsystem is not structured in a telecentric optical arrangement, variationin object surface height will cause a magnification error in addition toa defocused condition. As to the magnification error, correction can bemade through feedback control of a deflection circuit using heightvariation data, thus making it possible to always attain a chargedparticle beam image at the same magnification. Further, if themicrostructure observation/fabrication system using the convergentcharged particle beam is provided with a mechanism capable of moving anobject in the Z-axis direction with high accuracy and at response speedsufficient for focal point control, resultant data of height detectionmay be used for object stage height feedback control instead of feedbackcontrol of the charged particle beam optical system.

Where stage height feedback control is carried out, a surface of theobject can always be maintained at a constant height with respect to theheight detector and the charged particle beam optical system. Therefore,no problem will arise even if a guaranteed detection accuracy range ofthe height detector is narrow. As a drive mechanism for an object stage,there may be provided a piezoelectric mechanism enabling fine movementat high speed under vacuum, for example. When such a piezoelectricmechanism is used, a magnification error does not occur since a heightof the object surface is always maintained at a constant level withrespect to the charged particle beam optical system.

Calibration of objective lens control current and focal point in thecharged particle beam optical system may be carried out in the followingmanner. In an instance where there is a nonlinear relationship betweenobjective lens control current and focal point, it is required to makecorrection for nonlinearity. Linearity evaluation and correction valuedetermination may be effected as described below.

Referring to FIG. 21, there is shown a standard pattern 31 a forcalibration. As shown in FIG. 22, this standard calibration pattern issecured to a stage for holding an object. The standard calibrationpattern is made of conductive material so that it will not be charged byscanning of the charged particle beam. It is also desirable to providesuch a surface pattern feature that a height at each position can beidentified.

When the object holding stage is movable on a plane as in the inspectionsystem shown in FIG. 2, the standard pattern is moved to an observationregion at the time of calibration. Using the standard pattern, objectivelens control current measurement is effected to determine a currentlevel where a charged particle beam image becomes sharpest at eachpoint. At this step, visibility of the charged particle beam image isdetermined through visual observation or image processing. In thismeasurement, it is possible to determine a relationship betweenvariation in object surface height and optimum level of objective lenscontrol current as shown in FIG. 23. If the relationship betweenvariation in object surface height and optimum level of objective lenscontrol current is determined, a value of objective lens control currentwhich is most suitable for forming the charged particle beam image infocus can be identified using object surface height data attained by theheight detector.

The standard pattern 31 a shown in FIG. 21 has a flat part at both endsthereof. At each flat part, if a reference height is determined throughmeasurement with the optical height detector, gain/offset calibration ofobjective lens control current can be made according to heightmeasurement data. In case that characteristics of objective lens controlcurrent and focal point are calibrated for the objective lens by anymeans, gain/offset calibration of objective lens control current may bemade with respect to the optical height detector using a standardpattern 31 b which has two step parts as shown in FIG. 24.

Where the object holding stage is not provided with a movementmechanism, the charged particle beam optical system can be calibrated bydisposing the standard pattern so that it will always be located in avisual field of the charged particle beam optical system. Further, thestandard pattern may be formed so that it can be attached to an objectholding jig. Thus, even when the object holding stage is not providedwith a movement mechanism, it is possible to perform calibration bysetting the standard pattern on the stage and thereafter exchange thestandard pattern with the object for observation.

In case that the charged particle beam system is provided with amechanism for moving an object in the height direction as shown in FIG.25, an ordinary stepless pattern is utilizable instead of the standardpattern shown in FIG. 21. Through height detection by Z stage movementand image evaluation using the stepless pattern, calibration ofobjective lens control current can be made with respect to the heightdetector. Where there is provided a movement mechanism for Z stage, itis possible to conduct focus adjustment using the Z stage. However, if aresponse speed of the Z stage is not sufficiently high for anobservation region change speed, focal adjustment may be made using theobjective lens control current with the stage being fixed.

Calibration of the charged particle beam optical system using thestandard pattern shown in FIG. 21 is practicable only in amicrostructure observation/inspection system which allows observation ofa surface feature of the standard pattern using the charged particlebeam optical system. As contrasted, in a microstructure fabricationsystem, calibration is to be made only for the height detector using thestandard step-pattern shown in FIG. 24, and for a relationship betweenfocal point and control current of the charged particle beam opticalsystem, calibration is made beforehand therein. Where the microstructurefabrication system is provided with a charged particle beam imageobservation mode in which such an operational parameter as anaccelerating voltage for the convergent charged particle beam can bealtered, it is possible to check a point detected by the height detectorusing a charged particle beam image.

The following describes exemplary embodiments concerning focal pointcorrection and relationship between height measurement position underinspection and observation position in the charged particle beam opticalsystem. If the observation position of the charged particle beam opticalsystem completely meets the height detection position of the heightdetector, focus adjustment may be made according to height data detectedby the height detector. However, in the light-reflected positiondetecting method, a deviation of detection position occurs due tovariation in object surface height as illustrated in FIG. 3. Designatinga predictable value of maximum variation in object surface height asZmax and an incidence angle in the height detection optical system as θ,a value of maximum positional deviation Xmax is equal to Zmax·tan φ.Then, on condition that a value of allowable variation in object surfaceheight in terms of focal depth of the charged particle beam opticalsystem and performance requirement for the system is z0 and apredictable value of maximum gradient of object surface is Δmax, a valueof height detection error for maximum positional deviation dz isexpressed as Δmax·Xmax=Δmax·Zmax·tan θ as indicated in FIG. 26. If theheight detection error dz is smaller than z0, there arises no problem.However, if dz is larger than z0, it is required to attain a height onthe optical axis of the charged particle beam optical system.

In the inspection system according to the present invention, sincecontinuous inspection is performed by moving the stage, height data ateach point can be attained continuously. Using resultant data of heightdetection, a height of object surface in an observation region in thecharged particle beam optical system may be presumed or predicted toenable focus adjustment. Focus adjustment when there is a positionaldeviation between the height detection position and the observationregion in the charged particle beam optical system may be effected inthe following manner. In the following description, it is assumed thatstage scanning is performed by deflecting the beam of the chargedparticle beam optical system in the Y-axis direction and moving thestage in the X-axis direction to produce a two-dimensional image.

Where each of X-axis and Y-axis stage scanning movements is alwayslimited to one direction at the time of inspection, if each of theX-axis and Y-axis scanning movements is always made in one directiononly as shown in FIG. 27, i.e., reciprocal scanning movement is notperformed, the height detector may be disposed with an offset so thatthe height detection position will always be located before theobservation position of the charged particle beam optical system withrespect to the direction of stage scanning movement as shown in FIG.27(a). In this manner, a height at a desired position can be determinedusing height data in the vicinity of the observation region, which isattainable before each step of inspection.

As shown in FIG. 27(b), three points in the vicinity of the currentinspection position are selected and a height of the inspection positionis presumed according to a local plane determined by these three points.It is necessary to select three points so that the current inspectionposition will be located inside a triangle formed with the selectedthree points. Thus, a height of the inspection position can be presumedreliably through interpolation. In this case, although a height of astage scanning position at the start of inspection cannot be presumed,it can be determined by performing a sequence of scanning for heightdetection in advance.

Another exemplary embodiment is considered in that either one of X-axisand Y-axis stage scanning movements is always limited to one directionand also the axis movable only in one direction coincides with theprojection direction of the height detection optical system. As shown inFIG. 28, if the X-axis stage scanning movement is always limited to onedirection and the X axis coincides with the projection direction of theheight detection optical system, positional deviation in heightdetection due to variation in height takes place only in the X-axisdirection. Therefore, by providing an offset in the X-axis direction asshown in FIG. 28(a), a height can be determined through one-dimensionalinterpolation using height data on one line only. In this case, a heightof the inspection position may be determined by means of linearinterpolation using two-point data or spline interpolation usingthree-point data. At the start of inspection, a height detection valuein an entrance section until the stage reaches a constant speed may beused.

Further, as shown in FIG. 29, if the Y-axis stage scanning movement isalways limited to one direction and the Y axis corresponds to theprojection direction of the height detection optical system, positionaldeviation in height detection due to variation in height takes placeonly in the Y-axis direction. Therefore, by providing an offset in theY-axis direction as shown in FIG. 29(a), a height of the inspectionposition can always be determined reliably through interpolation usingheight detection data on a preceding line. In case that the stage ismoved in a reciprocal scanning fashion, such an offset as mentionedabove cannot be provided in one direction.

In an arrangement that the optical axis of the charged particle beamoptical system is made to coincide with a reference position of heightdetection, it is possible to presume a height of the inspection positionusing height detection data attained. However, since a height of theinspection position cannot always be determined through interpolation,its reliability is not ensured. For reliable height detection, there maybe provided such an arrangement that the height detection optical systemis equipped with a movable mechanism and the entire optical system isshifted in parallel as shown in FIG. 30 so as to give an offset in thestage scanning movement direction. Thus, a height of the inspectionposition can always be determined reliably through interpolation in thesame manner as in the foregoing example. There may also be provided suchan arrangement that a plurality of height detectors are disposed toenable height measurement at a plurality of points in the vicinity ofthe inspection position. In this arrangement, data of only necessarypoints can be used according to the stage scanning movement direction.

Exemplary embodiments for optical height detection in which a height ofa specimen surface can be detected reliably without being affected by astate of the specimen surface are now considered. In case that aspecimen surface height is detected by the light-reflected positiondetecting method as shown in FIG. 3, a deviation of a detection positionoccurs to cause an error in height detection. As shown in FIG. 31, if aspecimen surface 32 is provided with pattern areas having differentreflectances (high reflectance area 36, low reflectance area 37) andslit light is projected onto a pattern boundary 38 therebetween,reflected light intensity distribution 34 of slit light to be detectedis affected to cause an error in height detection. Such a heightdetection error may be reduced in the following manner. As shown in FIG.32, two slit light beams are projected onto the specimen surface indirections symmetrical with respect to a normal line thereon, andrespective reflected light beams from the specimen surface are detected.If sensors for detecting these slit light beams are disposed as shown inFIG. 32, a light image shift due to variation in specimen surface heightis made in the same direction and a measurement error due to specimensurface features appears in the opposite directions. Therefore, aneffect of specimen surface pattern features can be canceled by means ofaddition. Further, in case that the slit light beams are projected intwo directions as shown in FIG. 32, a deviation of the detectionposition due to variation in height occurs to the same extent in theopposite directions. Therefore, a deviation of the detection positioncan be eliminated by means of averaging.

FIG. 33 shows a method for reducing an effect of specimen surfacepattern features using a plurality of fine slits. A height detectionerror due to specimen surface pattern features increases in proportionto a slit width. Therefore, as shown in FIG. 33(a), a plurality of fineslit light beams are projected onto the specimen surface, and reflectedlight beams are detected by a linear image sensor. Individual centervalues of plural slit beam images are determined and averaged, thusmaking it possible to reduce an error in height detection. As shown inFIG. 33(c) in comparison with FIG. 33(b), an error on a pattern boundarycan be reduced by decreasing each slit width. Since fine slit beams onother than the pattern boundary are not affected by pattern features, anerror on the pattern boundary can be decreased through averaging.Although the quantity of light to be detected decreases as each slitwidth is decreased, an S/N ratio can be improved by averaging for pluralslit positions, thereby ensuring reliability in height detection.

According to the present invention, it is possible to detect a height ofan observation position in the electron beam optical system using theoptical height detector and attain an in-focus electron beam image whileconducting inspection. In an electron beam inspection system, inspectionperformance and reliability thereof can be improved by carrying outinspection using an electron beam image which is always focused in aconsistent state. Furthermore, since height detection can be madesimultaneously with inspection, continuous stage movement is applicableto inspection to reduce a required inspection time substantially. Thisfeature is particularly advantageous in inspection of semiconductorwafers which will become still larger in diameter in the future.Similarly, the same advantageous effects can be attained in amicrostructure observation/fabrication system using a convergent chargedparticle beam. Further, by disposing the height detection optical systemoutside the vacuum specimen chamber, adjustment and maintenance can becarried out with ease.

While we have shown and described several embodiments in accordance withthe present invention, it is understood that the same is not limitedthereto but is susceptible of numerous changes and modifications asknown to those skilled in the art, and we therefore do not wish to belimited to the details shown and described herein but intend to coverall such changes and modifications as are encompassed by the scope ofthe appended claims.

1. A convergent charged particle beam apparatus comprising: an electronbeam system which emits a converged electron beam; a vacuum chamberwhich is connected to the electron beam system; a stage which mounts aspecimen and moves at least in one direction inside of the vacuumchamber; an electron beam image observation unit which observes anelectron beam image of a surface of the specimen mounted on the stage byirradiation of the electron beam emitted from the electron beam systemand scanned over the surface of the specimen; a height detector whichoptically detects a height of the specimen mounted on the stage byilluminating the surface of the specimen with light and by detectingreflected light of the illumination reflected from the surface of thespecimen; and a controller which controls a focus position of theelectron beam in accordance with an output from the height detectorutilizing calibration data based on a relation between a focal point anda control current of the electron beam system while the stage is movingat least in one direction.
 2. A convergent charged particle beamapparatus according to claim 1, further comprising an image processingunit which receives the electron beam image from the electron beam imageobservation unit and processes the received electron beam image.
 3. Aconvergent charged particle beam apparatus according to claim 1, whereinthe height detector obliquely illuminates the surface of the specimenwith light and detects light reflected from the specimen with a sensor.4. A convergent charged particle beam apparatus according to claim 3,wherein the specimen has a pattern formed thereon, and the lightilluminating the surface of the specimen has a slit shape.
 5. Aconvergent charged particle beam apparatus according to claim 1, whereinthe calibration data is obtained utilizing a standard step-pattern.